1. Field of the Invention
The present invention relates generally to tooth rolling flat dies for forming teeth, such as spline teeth and gear teeth on the periphery of cylindrical workpieces, and in particular to tooth rolling flat dies and a method for forming the teeth which are adapted for forming the teeth by semi-dry forming in which a very small amount of oil mist is sprayed on the workpieces to be rolled.
2. Description of the Related Art
Tooth rolling flat dies are known means for forming teeth, such as spline teeth and gear teeth on the periphery of cylindrical workpieces, and are generally rectangular plate members having one working surface thereon toothed. These teeth include alternate ridges and grooves, which conform in plan to the shape of the teeth to be rolled. The teeth run the length of the die face. The dies are used in mating pair with their toothed working faces facing each other. A cylindrical workpiece of the teeth to be rolled is placed between the dies at one end thereof. The dies are then pressed against the cylindrical workpiece and the dies is moved longitudinally, thereby rotating the workpiece about its axis and roll forming the teeth thereon while the workpiece is at ambient or room temperature and without removal of material from the workpiece.
Conventional tooth rolling flat dies are shown, for example, in U.S. Pat. No. 4,862,718 in which each of the working face having a start, which is a chamfer with depth equivalent to thread depth, and a ramp region on the die to permit the die to penetrate the cylindrical blank at a controlled rate until full depth is reached. These dies have also a dwell portion or finishing portion having full ridge height and a roll-off section which permit gradual loss of contact between the die and workpiece, without marking the finished workpiece.
Also, in Japanese examined utility model publication Hei 6(1994)-200 discloses in its FIG. 4 which is reproduced in FIG. 3 PRIOR ART of present application. Each die face of the tooth rolling flat dies 11(12) shown in the FIG. 3 is provided thereon in series a leading teeth section 13 wherein height of the tops of the teeth 7 are gradually increasing from the front edge C to the rear edge D of the die until full height 3f thereof is reached to permit the die to penetrate the cylindrical workpiece until full depth is reached, a finishing teeth section 14 in which each of the teeth having full ridge height and a roll-off teeth section 15 in which height of the tops of the teeth are rapidly decreasing to permit gradual loss of contact between the die and the workpiece. Each of the lengths of the leading teeth section 13 and the finishing teeth section 14 is made to have that of at least more than one revolution(s) of the cylindrical workpiece.
Usually, in the tooth rolling flat dies 11(12) shown in the FIG. 3, the former substantially half portion 13a of the leading teeth section 13 is applied thereon a sandblasting treatment to prevent a slip between the die and the workpiece. Since in each of the latter half portion 13b of the leading teeth section 13, the finishing teeth section 14 and the roll-off teeth section 15, the teeth of each die and the workpiece mesh with each other closely, no slip occurs therebetween. Therefore, no sandblasting treatment is applied in these sections. From the point of view of machining accuracy of the machined surface, it is essentially important that no slip between the die and the workpiece occurs. Therefore, it is common practice to apply the sandblasting treatment on the former substantially half portion 13a of the leading teeth section 13 so that a frictional force is generated between it and workpiece, thereby prevents the slippage when the die penetrates the cylindrical workpiece. For this aim, the surface roughness Rz of maximum height of the former substantially half portion 13a of the leading teeth section 13 is made to range 20 to 35 xcexcm to permit so that no slippage occurs when oil coolant is applied on the workpiece.
Heretofore, rolling teeth for forming teeth, such as spline teeth and gear teeth on the periphery of cylindrical workpieces are generally conducted by pouring oil coolant on the workpiece. However, from the points of view of the protection of environment, for clean machining and for saving oil coolant cost, tooth rolling flat dies and a method for forming the teeth without using oil coolant, that is forming the teeth by semi-dry forming in which a very small amount of oil mist is sprayed on the cylindrical workpiece to be rolled, is desired. To date, however, semi-dry forming using conventional tooth rolling flat dies resulted that the lives of the dies are shortened compared with those of the dies using oil coolant thereby increased the machining cost.
Accordingly, it is a general object of the present invention to provide tooth rolling flat dies and a method for forming the teeth, such as spline teeth and gear teeth on the periphery of cylindrical workpieces, which overcome the disadvantages of the prior art and which have the same service lives as those of the conventional tooth rolling flat dies using oil coolant even if a semi-dry forming the teeth in which a very small amount of oil mist is sprayed on the workpieces to be rolled is conducted, thereby enabled to provide an environment-protective tooth rolling flat dies for semi-dry forming the teeth.
According to one aspect of the present invention, these and other object of the present invention are achieved by providing a tooth rolling flat die for forming a tooth including a spline tooth and a gear tooth on a periphery of a cylindrical workpiece. Each die having one working surface thereon teeth sections comprising in series a leading teeth section wherein heights of the tops of the teeth are gradually increasing from a front edge to a rear edge of the die until full height thereof is reached, a finishing teeth section each of the teeth having the full height and a roll-off teeth section in which heights of the teeth are rapidly decreasing. Wherein the first substantially quarter length portion of the leading teeth section has a first surface roughness Rz of maximum height of ranging 20 to 35 xcexcm, and the second substantially quarter length portion of the leading teeth section has a second surface roughness Rz of maximum height of ranging 5 to 20 xcexcm.
From the applicants"" study, they discovered that a typical pattern to end the service lives of many conventional tooth rolling flat dies is followingly, that is, at first, a chipping off from the edge of the coast side (the finishing teeth section side as seen from the leading teeth section) of a tooth crest of the second substantially quarter length portion of the leading teeth section occurs, then, the pressure load on the teeth of the finishing teeth section increases, resulting that the teeth thereof are destroyed and ends the final service life of the die. Also, applicants discovered that the reason that the chipping occurs firstly at the edge of the teeth of the second substantially quarter length portion on which the sandblasting treatment is applied lies in that, since the surface roughness of the teeth is coarse or rough, a stress concentration is liable to cause thereon. Further, a large frictional force is generated on the teeth which results the corresponding increase in the stress applied thereon. Additionally, applicants further observed that the slippage decreases in the former substantially half portion of the leading teeth section when the semi-dry forming is conducted.
As stated earlier, the surface roughness Rz of maximum height of the teeth of the former substantially half portion 13a of the leading teeth section 13 of the conventional tooth rolling flat die 11,(12) is made to range 20 to 35 xcexcm by applying a sandblasting thereon. However, according to the present invention, since the surface roughness Rz of maximum height of the teeth of the second substantially quarter length portion of the leading teeth section is made smaller to range 5 to 20 xcexcm, the stress concentration thereon is decreased which results the corresponding decrease in the stress thereon. This prevents the occurrence of the chipping off from the teeth of second substantially quarter length portion of the leading teeth section in which such a chipping off is liable to occur in the conventional tooth rolling flat die. Further, although the surface roughness Rz of maximum height of the teeth of the substantially quarter length portion is made smaller, in case the semi-dry forming is conducted, no slippage between the die and the workpiece occurs.
Since no slippage between the die and the workpiece occurs and the occurrence of the chipping off from the teeth of second substantially quarter length portion of the leading teeth section is very few, the tooth rolling flat dies for forming teeth according to the present invention enabled to have the same service lives and machining accuracy of the machined surface as those of the conventional tooth rolling flat dies using oil coolant even if the semi-dry forming the teeth is conducted, thereby it enabled to provide environment-protective tooth rolling flat dies for semi-dry forming the teeth. The surface roughness Rz of maximum height of the teeth is easily adjusted by selecting appropriate size of abrasive grits used in sandblasting treatment, and these adjustment is simple and easy to process.
According to another aspect of the present invention, the above and other object of the present invention are also achieved by providing a tooth rolling flat die for forming a tooth including a spline tooth and a gear tooth on a periphery of a cylindrical workpiece having one working surface thereon teeth sections comprising in series a leading teeth section wherein heights of the tops of the teeth are gradually increasing from a front edge to a rear edge of the die until full height thereof is reached, a finishing teeth section each of the teeth having the full height and a roll-off teeth section in which heights of the teeth are rapidly decreasing. Wherein the first substantially quarter length portion of the leading teeth section has a first surface treated thereon a first sandblasting treatment, and the second substantially quarter length portion of the leading teeth section has a second surface treated thereon a second sandblasting treatment, so that the surface roughness Rz of maximum height of the teeth of the first substantially quarter length portion of the leading teeth section is made larger than that Rz of maximum height of the teeth of the second quarter length portion.
Preferably, the surface roughness Rz of maximum height of the teeth of the first substantially quarter length portion ranges 20 to 35 xcexcm, whereas that Rz of maximum height of the teeth of the second substantially quarter length portion ranges 5 to 20 xcexcm.
More preferably, high hardness of the surfaces of teeth of the tooth rolling flat dies is preferred, to achieve this, an ion-nitriding process in which in a vacuum chamber the dies are connected to a negative pole and the wall of the vacuum chamber is connected to a positive pole, wherein a nitrogen gas and a hydrogen gas are introduced therein to be effected a glow discharge, thereby a nitriding is coated on the surfaces of dies. A shot peening process or both ion-nitriding and the shot peening process may be applied on the whole or partial surfaces of the die. Applying the ion-nitriding process first before the shot peening process is preferred, however, the ion-nitriding process may be followed to the shot peening process. The tooth rolling flat dies thus both processed are adapted for forming the teeth by semi-dry forming in which a very small amount of oil mist is sprayed on the workpiece to be rolled. By applying such ion-nitriding and/or shot peening process on the die, a tooth rolling flat die and a method for forming the teeth which are adapted for forming the teeth by semi-dry forming having a longer service life are provided.